The Faiyum Oasis (or Al-Fayoum Oasis, Al Fayyum Oasis, and other variations in spelling) is a depression or basin in the desert immediately to the west of the Nile south of Cairo. The extent of the basin area is estimated at between 1,270 km² (490 mi²) and 1700 km² (656 mi²). The basin floor comprises fields watered by a channel of the Nile, the Bahr Yussef, as it drains into a desert depression to the west of the Nile Valley. The Bahr Yussef veers west through a narrow neck of land north of Ihnasya, between the archaeological sites of El-Lahun and Gurob near Hawara; it then branches out, providing rich agricultural land in the Faiyum basin, draining into the large saltwater Lake Moeris (Birket Qarun). The lake was freshwater in prehistory but is today a saltwater lake. It is a source for tilapia and other fish for the local area.